The relation of dimer size, double-stranded, circular DNA of the replicative form of the bacterial viruses phi X174, S13 and G4 to genetic recombination is being investigated in continuation of our previous work on this subject. A dimeric intermediate, the "figure-8", has been identified by electron microscopy that consists of one dimer-length and two monomer-length circular strands in the form of a covalently-closed duplex. The figure-8 is a configuration in which branch migration can occur. This process has been shown to be obligatory for recombination in the phi X-G4 system. The properties of the figure-8 are being studied by biochemical, genetic and physical methods including electron microscopy and analytical ultracentrifugation. These studies will make use of isolated figure-8s and their infectivity in spheroplast tests of recombination. They will attempt to define the in vivo role of the figure-8 in recombination and its conversion or other relation to circular and catanated dimers. A configuration of DNA derived from the figure-8 of phage G4 by treatment with the Eco R1 restriction enzyme has made available a DNA in which the kinetics of branch migration are being measured and factors evaluated that determine the rate and significance of the process in vivo. BIBLIOGRAPHIC REFERENCES: Thompson, B.J., Camien, M.N. and Warner, R.C. Kinetics of Branch Migration in Double-stranded DNA. Proc. Nat. Acad. Sci. U.S.A. 73, 2299-2303 (1976). Kolodner, R., Tewari, K.K. and Warner, R.C. Physical Studies on Covalently Closed Circular Chloroplast DNA from Higher Plants. Biochim. Biophys. Acta. 447, 144-155 (1976).